Zirconium oxide ceramics are characterized by excellent biocompatibility and outstanding mechanical properties, which is why in the past they have increasingly been used as a material for implants and prostheses, but also as framework materials for dental restorations. Ceramics based on partially stabilized zirconium oxide are primarily used for this.
In many cases it is desirable to alter the surface of the zirconium oxide ceramic by coating it with a different material. Specifically when preparing dental restorations based on zirconium oxide ceramic, such a coating is regularly used to give the restoration the desired visual properties.
Glass ceramics have already been used in the past to coat or veneer oxide ceramics, such as zirconium oxide ceramics. These include feldspar-based ceramics or fluoroapatite glass ceramics.
Lithium disilicate glass ceramics are also known which, because of their high translucency and very good mechanical properties, are used in particular in the dental field and primarily for preparing dental crowns and small bridges.
EP 1 505 041 and corresponding U.S. Pat. No. 7,316,740, the entirety which is hereby incorporated by reference, describe lithium silicate glass ceramics which can additionally contain 0 to 2 wt.-% ZrO2. These are processed into the desired dental restorations in particular in the form of lithium metasilicate glass ceramics by means of CAD/CAM methods, wherein a subsequent heat treatment effects the conversion of the metasilicate phase to the high-strength disilicate phase. The glass ceramics can also be used for pressing over ceramic restorations.
EP 1 688 398 and corresponding U.S. Pat. No. 7,452,836, the entirety which is hereby incorporated by reference, describe similar lithium silicate glass ceramics which are substantially free of ZnO and in addition to other components can contain 0 to 4 wt.-% ZrO2. To achieve high strengths, however, small quantities of from 0 to 2 wt.-% ZrO2 are preferred. These glass ceramics also serve in particular to prepare dental restorations after mechanical processing by means of CAD/CAM.
However, these lithium silicate glass ceramics known from the state of the art have the disadvantage that they are not suitable for coating zirconium oxide ceramic in particular by means of a pressing-on in the viscous state, since after the pressing-on in the viscous flow process cracks and flaws form in the glass ceramic. Thus, such a composite does not have the mechanical properties that are indispensable specifically for use as dental restoration material.
Glass ceramics with lithium disilicate as main crystal phase which are to be suitable for veneering dental restorations comprising yttrium-stabilized zirconium dioxide are also known from WO 2008/106958 and corresponding U.S. Published Application No. 2011030423, the entirety which is hereby incorporated by reference. However, these glass ceramics contain quantities of only up to 6.0 wt.-% of ZrO2 and substantial quantities of Na2O. The ZrO2 present acts merely as a standard nucleating agent together with an optionally present further nucleating agent such as TiO2 in order to effect the formation of the desired lithium disilicate crystal phase.